[Numpy-discussion] matrix wart

Alan G Isaac aisaac@american....
Thu Feb 21 18:10:24 CST 2008


> On Thu, Feb 21, 2008 at 12:08:32PM -0500, Alan G Isaac wrote:
>> a matrix behavior that I find bothersome and unnatural::

>>     >>> M = N.mat('1 2;3 4')
>>     >>> M[0]
>>     matrix([[1, 2]])
>>     >>> M[0][0]
>>     matrix([[1, 2]])


On Fri, 22 Feb 2008, Stefan van der Walt apparently wrote:
> This is exactly what I would expect for matrices: M[0] is 
> the first row of the matrix.

Define what "first row" means!
There is no standard definition that says this is means the
**submatrix** that can be created from the first row.
Someone once pointed out on this list that one might 
consider a matrix to be a container of 1d vectors.  For NumPy, 
however, it is natural that it be a container of 1d arrays.
(See the discussion for the distinction.)

Imagine if a 2d array behaved this way.  Ugh!
Note that it too is 2d; you could have the same 
"expectation" based on its 2d-ness.  Why don't you?

You "expect" this matrix behavior only from experience with 
it, which is why I "expect" it too, while hating it. It is 
not what new users will expect and also not desirable.
As Konrad noted, it is very odd behavior to treat a matrix 
as a container of matrices.  You can only "expect" this 
behavior by learning to expect it (by use), which is 
undesirable.

Nobody has objected to returning matrices when getitem is 
fed multiple arguments: these are naturally interpreted as 
requests for submatrices.  M[0][0] and M[:1,:1] are very 
different kinds of requests: the first should return the 0,0
element but does not, while M[0,0] does!  Bizarre!
How to guess??  If you teach, do your students expect this 
behavior?  Mine don't!

This is a wart.

The example really speaks for itself. Since Konrad is an 
extremely experienced user/developer, his reaction should 
speak volumes.

Cheers,
Alan Isaac





More information about the Numpy-discussion mailing list